Method of testing electromechanical filters



May 26, 1959 R. L. S'HARMA METHOD OF TESTING ELECTROMECHANICAL FILTERSFiled Aug. 51, 1954 SIGNAL GENERATOR ATTORNEY Electromechanical filterssuch as desci-ibed in Patent No. 2,656,516 are becoming of increasingimportance. to a the electrical industry for the reason that they I arevery selective and obtain substantially better resultsithanelectricalcomponents.

Techniques of manufacturing suchv filters have-been described in thecopending patent applications ofFrank r :1

C. Wallace entitled Assembly Jig, Serial No. 253,575, filed October 29,1951, now Patent No. 2,716,178; End Wire Frequency Measuring and CuttingJig, Serial No. 276,398, filed March 13, 1952, now Patent No. 2,690,803;Disc Holding Frequency and Measuring Device, Serial No. 284,082, filedApril 24, 1952, now Patent No. 2,734,379; Disc Sander, Serial No.292,277, filed June 7, 1952, now Patent No. 2,715,802; and patent onDisc Edge Deburring Machine, Patent No. 2,650,458, issued September 1,1953, to Frank C. Wallace. The general problem presented, however, is toobtain a plurality of discs which are tuned to a desired frequency andconnect them by coupling wires with the proper response characteristics.Each end disc is connected to an end wire which is tuned for the desiredresults.

In practice the discs are tuned separately before being assembled. Thenthe end discs are tuned with the end wires before the filter isassembled. However, even though great care is taken in the assembly ofthe parts, it sometimes happens that after the filter has been assembledthe over-all response is bad. It may be that an end wire or perhaps oneof the discs is out of tune, but it is very diflicult to determine whichpart is out of tune. This present invention relates to a method fordetermining the response of various portions of the mechanical filtersso as to allow a determination to be made as to which disc or Wire isout of tune.

Further objects, features, and advantages of the invention will becomeapparent from the following description and claims when read in view ofthe drawings, in which:

Figure 1 is a side view of the damping tool of this invention,

Figure 2 is a perspective view of the invention,

Figure 3 illustrates the damping tool in use with a mechanical filter,

Figure 4 illustrates a modification of the invention, and

Figure 5 is a partial sectional view of Figure 4.

Figure 1 illustrates the damping tool of this invention, and itcomprises a handle 10 which might be made of a suitable plastic, forexample, into which is inserted a blade 11. The blade 11 may be made ofmetal, or of any other suitable material.

A spring clamp 12 is attached to the handle 10 at its center and has apair of legs, 13 and 14, for clamping the discs of the mechanicalfilter. The width of the legs 13 and 14 is great enough to span twodiscs in an electromechanical filter.

In use, the handle 10 is used to place the legs 13 and 14 over a pair ofdiscs 16 and 17 forming a part of a mechanical filter assembly. It is tobe noted that a number of other discs, 18 and 19, may be in the filterassembly.

2,887,877 Patented May 26, 1959 The discs are connected by couplingwires 21. The end disc 16 has a driving wire 22 attached thereto, whichmay =be connected to a magnetostrictive driving means.

Before placing the clamp over the discs 16 and 17 a liquid placed onboth sides of the blade 11 so that when the blade passes between thediscs 16 and 17 a thin layer of liquid will exist between each disc andthe blade.

The blade 11 is centered with respect to discs 16 and 17. It has beenobserved that the discs 16 and 17 will be completely damped out by theliquid 23 between them and the blade.

This may be explained as follows:

Suppose that the liquid is water, and that the thickness of waterbetween each side of the blade and the disc is designated by 11.. Assumethat the disc 16 is vibrating and that the blade 11 is not. There willbe a steady motion of water under pressure between the two fixed plates.

The equation for the state of equilibrium is bz Ba; (1)

Where u is the velocity of liquid particles, a is the coefficient ofviscosity of the liquid, p is the pressure, x is the distance along anaxis parallel to the plane of the discs, and z is the distance along theaxis transverse to the plane of the discs.

Solving Equation 1, which is a partial differential equation of thesecond order, may be accomplished as follows:

Solving for A and B when z is variable,

l 2 'F P2 'uz(h ham or L am-. am (4) Further simplification results inis the damping factor R which appears in the equation for forcedvibration as follows T a a; z t+Rs- 0 4 w 1 and r is the variable radiuson the circular plate, so

When R is made very large, the vibrations can be completely damped. From(7) it is obvious that R can be made very large by having h" very small(of the order of .005 inch in our case).

The usefulness of this tool (1) With this positive damping technique, itis now possible to tune the end wire to the desired frequency veryeasily. The filter may be assembled and the damping tool placed over thefirst two discs to dampen them. The end wire may then be tuned becauseit is isolated dynamically from the discs.

(2) After the end wire is tuned the tool may be moved so that it engagesthe second and third discs to dampen them. Then the response of the endwire and first disc may be measured and adjusted until correct.

The response is measured by magnetostrictive means including a variablefrequency signal generator 29 that is controlled by knob 30. A couplingcoil 28 surrounds driving wire 22 and a biasing magnet 31 is mountedadjacent coil 28. Wire 22 is made of magnetostrictive material and isexcited by alternatingcurrents in coil 28.

A coupling coil 26 is connected to a meter 27 and indicates theamplitude of vibration in wire 22 induced by coil 28. A biasing magnet32 is mounted adjacent coil 26.

The disc may be tuned by removing a part with a small grinder and theend wire may be tuned by cutting it to the correct length. The tool maybe moved from disc to disc until the entire assembly has been tested andcorrected.

At times it becomes desirable to tune the end wire of an end disc and anend wire assembly while disassembled from the filter. This may beaccomplished with the modification shown in Figures 4 and 5. A block 35is formed With a hole 36 into which the disc 37 may be received. Adriving wire 38 extends upwardly from disc 37. A clip 39 extends fromblock 35 to engage the disc. Liquid 41 is placed in the hole 36 anddampens disc 37 as previously described. The coils 26 and 28 andassociated driving and indicating circuitry are not shown but are thesame as in Figure 3. The frequency of the end wire may then bedetermined.

I claim:

A method of successively measuring at a desired frequency the responseof individual elements of an electromechanical filter that has aplurality of serially connected resonant elements comprising, applying adamping tool to said filter for damping a resonant element which, inrelation to a source for inducing mechanical vibrations of said desiredfrequency to said elements, succeeds that one of said elements of whichthe response is to be measured, said damping tool having a surface inclose proximity to the surface of said succeeding resonant element formaintaining a thin layer of liquid between the surface of saidsucceeding resonant element and the adjacent surface of said dampingtool whereby said succeeding element is dynamically isolated from saidone element that is connected to said vibration source, measuring theamplitude of the vibrations induced into said one element, and tuningsaid one element to obtain the desired response at said desiredfrequency.

References Cited in the file of this patent UNITED STATES PATENTS1,431,868 Bedell Oct. 10, 1922 1,562,543 Cox Nov. 24, 1925 1,942,323Blodgett Jan. 2, 1934 2,439,739 Hussman Apr. 13, 1948 2,526,211 EricksonOct. 17, 1950 2,692,681 Douglas Oct. 26, 1954

